1. Field of the Invention
The present invention relates generally to method and apparatus for treating the surface of a support of a lithographic plate, and more particularly to reducing the abrasion of a graining brush that grains the surface of an aluminum support of the lithographic plate.
2. Description of Related Art
The aluminum plate has widely been used as the support of a photosensitive printing plate or the lithographic plate. The surface of the aluminum support is grained in order to improve the adherence between a sensitive layer and the aluminum support and to provide non-image parts on the plate with the capacity to retain moisture.
Japanese Patent Provisional Publication No. 53-123204 discloses a brush graining method, which is now widely used as the surface treatment method for graining the surface of the aluminum support. In the brush graining method, a graining brush, which is made of a material such as nylon, rubs the surface of the aluminum support with abrasive slurry, which contains abrasive particles of materials such as pumice or pumicite, aluminum hydroxide, or alumina, which have new Mohs'scale of 2 or more. According to the brush graining method, it is possible to sequentially obtain the support that has a good printing performance, and the equipment costs can be relatively low. U.S. Pat. No. 4,477,317 discloses the brush graining method in which the graining brush oscillates while it rotates.
The widths of aluminum webs to be the aluminum supports are not uniform, and one graining brush must continuously rubs the surfaces of the narrow aluminum web and the wide aluminum web. However, according to the surface treatment apparatus of Japanese Patent Provisional Publication No. 53-123204 and U.S. Pat. No. 4,477,317, the bristles of the graining brush are abraded sectionally if the surfaces of the aluminum webs with different widths are grained with the one graining brush continuously. Hence, the graining brush must be replaced by a new one frequently, or the bristles of the graining brush must be trimmed frequently in accordance with the abrasion, and therefore, the surface graining is neither economical nor efficient.
Specifically, since the bristles are abraded at the central part of the graining brush, with which the aluminum webs come into contact, the aluminum webs with different widths must be transported to the surface treatment apparatus sequentially in order of the width of them. Consequently, the bristles become short at the central part of the used graining brush and become relatively long at both ends thereof. To treat the surface of the wide aluminum web again, the graining brush must be replaced by a new one, or the bristles of the graining brush must be trimmed so that they can be uniform length at the central part and both ends of the graining brush.